1. Field of the Invention
This invention relates to a variable light amount type electronic flash device, and, more particularly, to a device for flash light photography capable of extending the flash light photography responsive distance to the long distance side when a high speed film is used.
2. Description of the Prior Art
Conventionally, what is known as the adjustable light flash device (hereinafter referred to as the flash device) either attached to or incorporated in a camera may be divided into three main groups: (1) Illumination light from the stroboscopic device is projected to an object being photographed, and its reflected light is received either by the stroboscopic device or by the camera. When the amount of light integrated has reached a predetermined value, the production of light is automatically stopped. (2) The timing circuit for stopping the light production is previously adjusted in response to the setting of a distance between the stroboscopic device and the object so that the lighting of the stroboscopic device is caused to stop in correspondence to the set distance. (3) Before the lighting of the stroboscopic device, a preliminary lighting is performed. By the amount of light reflected, information of the object distance is obtained. In correspondence to this information, the energy of light emitted is adjusted.
When such adjustable light strobo devices are used, there gives rise to a problem that the diaphragm of the camera must be set by hand in conformance to the value specified by the exposure calculation plate of that strobo. There is no need to mention that this operation is troublesome. In addition thereto, when various films of different speed are used, there is a high possibility to commit faulty setting of the diaphragm. Further, even when a film of the same speed, for example, only ASA100 is used, the specified diaphragm value varies, depending upon the type of strobo, some requiring F4 and other F5.6, for example, so that the faulty setting of the diaphragm becomes high in possibility. Furthermore, in order to give selectivity of the depth of field in photography, it is known to provide an adjustable light strobo constructed so as to be capable of selecting a number of specified diaphragm values. In this case, the operation becomes more complicated with increase in the possibility of faulty setting.
In other words, the conventional adjustable light type flash device, as shown in FIG. 1, is constructed so as to automatically adjust the amount of light emitted in accordance with the object distance to effect proper exposure, while a predetermined diaphragm value is specified. In the figure, the abscissa is the object distance and the ordinate is the amount of light emitted from the flash device. The curves, a, b, c and d, show the various types of flash devices and how the emitted amount of light varies (or has to be varied) in response to the object distance, depending upon the switched position of a flash device of the type in which a number of specified diaphragm values are available.
In other words, for example, the "b" type flash device is required to set the diaphragm at F5.6 when a film of ASA100 is used, and is constructed so as to be automatically controlled in the emitted amount of light in accordance with the object distance, in a manner such that for an object at a distance of 1 meter, the emitted amount of light becomes Po.times.2.sup.2 (corresponding to G. No. 5.6) and at a distance of 1.4 meter, Po.times.2.sup.3 (corresponding to G. No. 8).
On the other hand, in the "c" type flash device, it is required to use not F5.6 but F4, and the construction is such that the emitted amount of light is controlled to Po.times.2 (corresponding to G. No. 4) for an object at a distance of 1 meter. Conversely speaking, as this type is constructed to emit Po.times.2 (corresponding to G. No.4) at 1 meter, it is required to use F4.
As will be seen from the foregoing, because of the availability of a wide variaty of conventional adjustable light type flash devices of different amount of light emission, the operator has the difficulty of resetting the specified diaphragm value each time the flash device to be used and the film speed which will be described later are changed. If he or she makes an error in evaluating it, no better than improper exposure can be obtained.
Further, in the above-described "b" type flash device, when a film of ASA50 is used, though the distance is the same, it is required to adjust the diaphragm value not to F5.6 but to F4 in order to double the amount of light reaching the film. In such a manner, there is need to readjust the diaphragm in response to the speed of the film to be used. This point also is troublesome.
Furthermore, a flash device capable of selectively controlling emitted amounts of light, in other words, having a great number of specified diaphragm setting positions is known. With such device, upon setting to a particular position, it becomes a particular type flash device as selected from the types, a, b, c and d mentioned above, so that the change-over setting of the diaphragm of the photographing lens becomes more troublesome to increase the possibility of performing faulty operation.
As has been described above, the conventional adjustable light strobo device has a drawback that each time the film speed setting is changed over, the diaphragm value must be reset. For this reason, it can be considered that the setting of film speed and the setting of diaphragm value are coupled with each other. With such a flash device, when a high-speed film (for example, ASA400) is used instead of a generally accepted film (for example ASA100), the size of diaphragm aperture of the photographing lens is caused to change in response to the set value of film speed. Therefore, the photographing limit of the longer distance side in flash light photography can not be extended despite the use of the high-speed film.
In other words, even in the adjustable light type flash device, as shown in FIG. 1, the produced light amount has an upper limit (in the illustrated example, Po.times.2.sup.6) likewise as in the usual non-adjustable light type flash device. Further, because of the construction of the light adjusting part, it is natural that the lower limit of the produced light amount exists (in the illustrated example, Po). Therefore, with the "b" type flash device, or the selectively settable flash device when set to the "b" position, for example, the range of photographable distances is from 0.5 m to 4 m. By the way, when the film of ASA400 is used in combination with the flash device of the "b" type or set to the "b" position, though the object distance is the same, it is necessary to use not F5.6 but F11 so that the amount of light reaching the film surface is made as large as 1/4 times that when ASA100 is used. For this reason, when the setting of film speed is coupled with the setting of diaphragm to automatically perform setting operation of the diaphragm as mentioned above, the change from ASA100 to ASA400 causes the change of the diaphragm from F5.6 to F11. Therefore, even when a film having as high a speed as ASA400 is used, the diaphragm of the photographing lens is closed down to an extent corresponding to the increase in film speed so that the range of photographable distances remains unchanged as shown in the figure thereby giving rise to a problem that photography can not be made for a long distant object.
This fact when considered in connection with the fact that the combination of the general non-adjustable light type flash device with the high-speed film makes it possible to photograph a long distant object can be well understood to be a very serious drawback for the adjustable light type strobo.